Hello,
I just tried both to enable native colors and also switch to default theme, restarted gimp in between and same results.
I use the latest stable version: 2.1.8 Qt, not sure of the Qt version used in this one, when going into the plugin folder and looking at the Qt5Core.dll details proprieties in windows i see 5.8.0.0, same version for all other Qtxxx.dll’s i checked.
I can’t compile stuff myself but changing the QT version could be something to test if possible.
Thank you.